1. Field of the Invention
The present invention relates to semiconductor laser devices which have a waveguide structure with a diffraction grating for controlling-the longitudinal mode, like dynamic single-mode semiconductor laser devices such as a distributed feedback semiconductor laser and a distributed bragg reflector semiconductor laser, and also relates to semiconductor laser modules, rare-earth-element-doped optical fiber amplifiers and fiber lasers using the same.
2. Description of the Related Art
There are known distributed feedback (DFB) semiconductor lasers, distributed bragg reflector (DBR) semiconductor lasers and the like as a semiconductor laser (laser diode(LD)) realizing a dynamic single-mode oscillation. Any one of these laser diodes has a waveguide structure incorporating therein a diffraction grating with a wavelength selecting function. Waveguide structures based on a stepped refractive index profile, in general, comprise a waveguide layer having a higher refractive index sandwiched between cladding layers having a lower refractive index. Prior art technologies will be summarized with attention given to the location of the diffraction grating in the waveguide structure.
An example of a first prior art technology is disclosed in Japanese Unexamined Patent Publication JP-A 8-316566 (1996) in which a diffraction grating is formed at the interface between a waveguide layer and a cladding layer. FIG. 16 is a sectional view, taken along the resonance cavity of a DFB laser diode, of the art shown in the Publication. FIG. 17 is a schematic view of the refractive index profile, as viewed vertically, of the waveguide structure. In this prior art reference, an unevenness is provided at the interface between an upper waveguide layer 41 and an upper cladding layer 42 to form an index modulation diffraction grating 43. This diffraction grating 43 is formed in the following manner: crystal growth is performed up to the upper waveguide layer; subsequently, the unevenness is formed on the surface by ordinary two-beam holographic lithography process and wet etching process; and crystal growth is performed again to form the upper cladding layer and its succeeding layers, thereby burying the unevenness to complete the grating.
An example of a second prior art technology is disclosed in Journal of Lightwave Technology, Vol. 7, No. 12, pp. 2072-2077, 1989, xe2x80x9c1.3-xcexcm Distributed Feedback Laser Diode with a Grating Accurately Controlled by a New Fabrication Techniquexe2x80x9d, in which a diffraction grating is buried within a cladding layer. FIG. 18 is a sectional view, taken along the resonance cavity, of this art. FIG. 19 is a schematic diagram of the refractive index profile, as viewed vertically, of the waveguide structure. Within a cladding layer 53 of n-InP, a diffraction grating comprising a diffraction grating layer 52 of n-InGaAsP having a higher refractive index than the cladding layer is buried. In this art, crystal growth is performed to form a barrier layer 51 of n-InP and the diffraction grating layer 52 of n-InGaAsP; subsequently, the resultant stacked structure is subjected to a two-beam holographic lithography process and a wet etching process to form a plurality of trenches having a depth reaching the barrier layer 51, the trenches being oriented perpendicular to the resonance cavity to form a striped structure; and finally, this striped structure is covered with the cladding layer 53 of n-InP that is the same material as that of the barrier layer, thereby completing the diffraction grating 54.
The coupling efficiency of a buried diffraction grating is determined by the following factors: sectional configuration of the diffraction grating, thickness, distance between the diffraction grating and the center of the waveguide structure, refractive indices of the diffraction grating layer and the layer in which the diffraction grating layer is buried, and the like. The literature of the second prior art mentions some advantages of the art including: reduced influence on the guided mode due to the cladding layer and barrier layer of the same composition, higher thickness controllability of the diffraction grating layer in the crystal growth, and like merits.
The coupling efficiency of the unevenness-type diffraction grating like the first prior art technology can be designed by adjusting the factors such as configuration of the unevenness, depth, distance between the diffraction grating and the center of the waveguide structure, refractive index of each of the layers lying on and under the diffraction grating. However, the design of the waveguide structure, including the location of the waveguide layer/cladding layer interface, is largely limited by the guided mode configuration and the beam-divergence angle. Further, the material (refractive index) of the waveguide layer is also limited to keep satisfactory the crystal quality of a portion adjacent the crystal re-growth interface. For this reason, the number of factors based on which the coupling efficiency of the unevenness-type diffraction grating can be designed independently of the waveguide structure is small, and thus, the design freedom has been largely restricted. Furthermore, the depth of the unevenness formed by wet etching is required to be controlled uniformly and accurately so as to form the unevenness-type diffraction grating having the coupling efficiency in conformity with the design. It is, however, difficult to control the wet-etching depth with precision and, hence, difficult to secure the uniformity and reproducibility of the coupling efficiency.
On the other hand, the guided mode propagating within the waveguide structure based on a stepped refractive index profile is configured concentrated in the waveguide layer having a higher refractive index, while on the other hand the intensity of the guided mode is rapidly attenuated in a exponential function fashion within the cladding layer having a lower refractive index. Since the coupling efficiency is determined by the overlap between the guided mode and the diffraction grating, it is required that the diffraction grating buried within the cladding layer as in the second prior art technology be located with a higher precision to provide the diffraction grating with a predetermined coupling efficiency. For this reason, strict limitation is imposed on both the design and the manufacture, resulting in a limited allowance. In semiconductor laser diodes of which the oscillation wavelength is about 1 xcexcm or smaller, in particular, Al-containing materials such as AlGaAs are frequently used for the cladding layer having a lower refractive index. In the case of the diffraction grating buried in the cladding layer formed of such an Al-containing material, it is very difficult to clean a surface of the cladding layer that has been oxidized during the formation of the diffraction grating prior to the crystal re-growth. For this reason, the crystal quality of a portion adjacent the crystal re-growth interface may be deteriorated, resulting in a danger that the reliability of the resultant device is lowered.
As described above, there has been a strong demand for a waveguide structure to which Al-containing materials are applicable, and which has a diffraction grating offering a wider design freedom in terms of the coupling efficiency and a wider manufacture freedom.
Accordingly, it is an object of the present invention to realize a waveguide structure to which Al-containing materials are applicable, and which has a diffraction grating offering a wider design freedom in terms of the coupling efficiency and a wider manufacture freedom, thereby providing a dynamic single-mode semiconductor laser device easily with higher reproducibility, yield and reliability.
It is another object of the present invention to provide a semiconductor laser module capable of being easily and efficiently connected to an optical fiber amplifier, an optical fiber laser and the like having an optical fiber as a main component, and a rare-earth-element-doped optical fiber amplifier and a fiber laser capable of contributing to high-speed long-haul optical communication.
The invention provides a semiconductor laser device comprising:
an active layer;
upper and lower waveguide layers sandwiching the active layer therebetween;
upper and lower cladding layers sandwiching the active layer and the upper and lower waveguide layers therebetween; and
a current narrowing structure defining a current-injection region for injecting current to the active layer,
wherein a diffraction grating having a periodical structure in a resonance cavity direction is buried in any one of the waveguide layers, and the waveguide layer in which the diffraction grating is buried and the cladding layer adjoining to that waveguide layer forms an interface which is substantially flat in the resonance cavity direction.
According to the invention, as described above, the guided mode propagating within the waveguide structure based on a stepped refractive index profile is of a configuration such as to have peaks within the waveguide layers having a higher refractive index. As a result, the guided mode has a relatively gentle intensity distribution within the waveguide layers. By burying the diffraction grating within one of the waveguide layers it becomes possible to relax the limitations on the positioning precision of the diffraction grating thereby expanding allowances in design and manufacture. In addition, since buried diffraction gratings have an advantage that such parameters as the thickness of the diffraction grating layer and the distance from the center of the waveguide structure can be designed completely independently of the waveguide structure, it is possible to secure a widened design freedom.
In the fabrication of a semiconductor laser device, the growth of a crystal proceeds in a direction perpendicular to a surface of the crystal. Accordingly, if, for example, in the case where asperities having an inclined face are formed on the surface of the crystal, the crystal is grown retaining the asperities, the crystal growth directions of adjacent inclined faces would intersect each other and, hence, the growing surfaces of the crystal would collide with each other. As a result, crystalline defects are likely to be accumulated at a location where the adjacent inclined faces contact each other, especially at the bottom of dips. When such crystalline defects are accumulated in the waveguide layer, problems such as optical absorption loss are raised, so that the oscillation characteristics and reliability of the device are affected. For this reason, the present invention provides the feature that the waveguide layer in which the diffraction grating is buried and the cladding layer adjoining to that waveguide layer forms an interface which is substantially flat in the resonance cavity direction, so as to make the crystal growing surface flat in the process of burying the unevenness thereby avoiding the accumulation of crystalline defects. This makes it possible to enhance the oscillation characteristics and reliability of the device.
In the semiconductor laser device of the invention, it is preferable that the diffraction grating is buried within the upper waveguide layer and is present in at least a part of the current-injection region.
According to the invention, the provision of the diffraction grating within the upper waveguide layer minimizes the influence of the grating configuration on the configuration and characteristics of the active layer. More specifically, where the diffraction grating is buried within the lower waveguide layer, the active layer is positioned above the diffraction grating. In this case, it is possible that any transformation or defect of the crystalline structure that may occur in the crystal re-growth on the uneven surface affects the active layer. By locating the diffraction grating above the active layer, such a possible disadvantage can be avoided and, hence, a decrease in luminous efficiency can be avoided.
In the process of burying the unevenness formed on the structure surface, the surface configuration gradually becomes substantially flat with crystal growth. In the case of a device structure in which the interface between the waveguide layer in which the diffraction grating is buried and the cladding layer adjoining thereto is not flat in the resonance cavity direction, it is very difficult to make up the micro-configuration of the interface influencing the waveguide in conformity with the design. In contrast, the device structure in which the interface is flat in the resonance cavity direction, it is easy c to make up the interface configuration in conformity with the design.
The carrier density distribution in the active layer grows high in the current injection region, and therefore, the light intensity also grows high in the current injection region. For this reason, the feature of the invention that the diffraction grating is present in at least a part of the current injection region enhances the light-grating coupling efficiency and hence improves the stability of the longitudinal mode.
In the semiconductor laser device of the invention, it is preferable that the device oscillates in a transverse multi mode.
According to the invention, since the device is of a transverse-multimode waveguide structure in which a plurality of transverse modes are distributed in a horizontal direction which is perpendicular to the resonance cavity direction and parallel with the active layer, a higher output can be attained. In addition, a diffraction grating is formed in the current injection region, which enhances the coupling efficiency between the diffraction grating which is present in the current injection region and each transverse mode. The coupling efficiency is further enhanced by the provision of the diffraction grating buried within the waveguide layer. Thus, a higher-output, single longitudinal mode oscillation will result. It should be noted that the transverse-multimode waveguide structure in the current injection region may be constructed of an index guiding structure having a wide horizontal width or a large index difference between the inside and the outside of the current injection region, or a gain guiding structure.
In the semiconductor laser device of the invention, it is preferable that the current narrowing structure is located farther than the interface between the cladding layer and the waveguide layer from the active layer.
According to the invention, by locating the current narrowing structure farther than the cladding layer/waveguide layer interface from the active layer, especially in a region outside the waveguide layer, the guided mode is less influenced by the current narrowing structure, and the resultant device is of a gain guiding structure. With the gain guiding-type transverse-multimode waveguide structure, the diffraction grating substantially coincides due to a small difference in effective index between the transverse modes. Accordingly, the oscillation spectrum of the overall device resulting from superposition of all the transverse modes is narrow, and thus, a substantially single wavelength can be selected.
The current narrowing structure can be formed by any ordinary process usually used in the fabrication of semiconductor laser devices such as a semiconductor layer burying process or an ion implantation process to provide a semiconductor layer having a higher resistance. Examples of such processes include confinement of the current injection region using a striped electrode structure or a dielectric film pattern.
The width, defined by the current narrowing structure, of the current injection region is preferably 10 xcexcm or more. This feature allows the gain guiding-type transverse-multimode waveguide structure to be realized readily.
In the semiconductor laser device of the invention, it is preferable that a confinement factor of a guided mode confined within a waveguide region as a total of the waveguide layer and the active layer is 0.8 or more.
According to the invention, by increasing the confinement factor of the guided mode confined within the waveguide region as the total of the waveguide layer and the active layer to 0.8 or greater, the overlap between the guided mode and the diffraction grating can be expanded thereby enlarging the design range of the coupling efficiency. In addition, since buried diffraction gratings have an advantage that such parameters as the thickness of the diffraction grating layer and the distance from the center of the waveguide structure can be designed completely independently of the waveguide structure, it is possible to secure a wider design freedom. Note that the confinement factor of a guided mode can be calculated using an analyzing method for multi-layered slab waveguide structures (see Kenji Kono, xe2x80x9cFundamentals and Applications of Optical Coupling Systems for Optical Devicesxe2x80x9d, pp. 152-161, GENDAIKOGAKUSHA, 1998).
The waveguide layer in which the diffraction grating is buried is preferably formed of a semiconductor material free of Al. Examples of such preferable materials include GaAs, InGaAsP and InGaP.
It is preferred that a crystal re-growth interface protective layer which is an Al-free semiconductor layer adjoining to the diffraction grating be provided on the substrate side of the diffraction grating and be buried within the waveguide layer together with the diffraction grating. Preferred examples of such semiconductor materials include GaAs, InGaAsP and InGaP.
Even in the case where an Al-containing material is used for the cladding layers having a lower refractive index, the use of an Al-free material such as GaAs for the waveguide layers having a higher refractive index enables the structure surface oxidized during the diffraction grating forming process to be cleaned prior to the crystal re-growth. For this reason, the crystal quality of a portion adjacent the crystal re-growth interface can be kept satisfactory thereby ensuring the resultant device enjoying an improved reliability.
Even in the case where Al-containing materials are used for both the cladding layers and the waveguide layers, insertion of the crystal re-growth interface protective layer of an Al-free material leads to advantages as above. In the case of the crystal re-growth interface protective layer of GaAs, InGaAsP or InGaP, the refractive index of the protective layer can be adjusted to a value substantially equal to that of the waveguide layers, thereby minimizing the influence on the guided mode sufficiently.
In the semiconductor laser device of the invention, it is preferable that a carrier blocking layer is interposed between the active layer and either of the upper and lower waveguide layers, the carrier blocking layer having a band gap larger than that waveguide layer.
According to the invention, the provision of the carrier blocking layer having a larger band gap between the waveguide layer and the active layer enables the carrier blocking layer adjacent the active layer to prevent injected carriers from flowing into any layer having an opposite conductivity type, thereby realizing a more efficient oscillation. Further, since either an electron or a hole is solely present within the waveguide layer, it is possible to assuredly inhibit carrier recombination affecting the laser characteristics and reliability even though the waveguide layer is directly processed for crystal re-growth in order to bury the diffraction grating. This facilitates the burying of the diffraction grating in the waveguide layer at a location near the current injection path and the active layer. Further, since the guided mode is loosely confined within the thick waveguide layer, the optical intensity can be lowered in the active layer thereby enabling a higher output operation. Additionally, the guided mode is expanded to enable compatibility between a favorable radiation pattern and a lower radiation angle, while at the same time the positional allowance of the diffraction grating is expanded to widen the design freedom and the manufacture freedom.
The carrier blocking layer has a thickness such as to inhibit the outflow of carriers of an opposite conductivity type satisfactorily and not to disturb the guided mode. Specifically, the thickness of the carrier blocking layer is desirably 5 to 50 nm.
In the present invention, the diffraction grating comprises stripes extending perpendicular to the resonance cavity in a plane parallel with the substrate and aligned periodically in the resonance cavity direction. In a sectional view taken along the resonance cavity direction, each stripe may be quadrangular or triangular in configuration. Further, the diffraction grating layer of the striped configuration is preferably formed of an Al-free material in terms of protection against deterioration due to oxidation during the processing thereof.
Further, the diffraction grating may be located to form either a distributed feedback (DFB) semiconductor laser device or a distributed bragg reflector (DBR) semiconductor laser device.
The invention provides a semiconductor laser module comprising:
a semiconductor laser device as recited above;
an optical fiber receiving laser light from the semiconductor laser device; and
a holder securing the semiconductor laser device and a laser light introducing portion of the optical fiber.
According to the invention, since the use of the higher-output, single-mode oscillation semiconductor laser device makes it possible to transmit higher-output, single wavelength laser light through optical fibers, the semiconductor laser device is easily connected to an optical fiber amplifier or optical fiber laser with the result that enhancement and stabilization of outputs from these apparatuses can be realized.
The invention provides a rare-earth-element-doped fiber amplifier comprising: an optical fiber doped with a rare earth element, and any one of the above-mentioned semiconductor laser devices for use as a rare-earth-element pumping source.
According to the present invention, the use of the above semiconductor laser module in the rare-earth-element-doped fiber amplifier enables excitation at higher output and, in addition, fixes the pumped wavelength thereby ensuring stabilized amplified outputs.
The invention provides a fiber laser doped with a rare earth element, comprising any one of the above-mentioned semiconductor laser devices as an excitation light source.
According to the invention, the use of the semiconductor laser module in a fiber laser which is preferable for high-density wavelength division multiplex (DWDM) in optical communication systems makes it possible to pump in higher output, and additionally because the excitation wavelength is fixed, stabilized outputs can be obtained.